These techniques are applied to igneous rocks, and are normally seen as giving the time since solidification.
The flood buried a huge amount of carbon, which became coal, oil, etc., lowering the total C ratio in plants/animals/the atmosphere before the flood had to be lower than what it is now.
It does not give dates of millions of years and when corrected properly fits well with the biblical flood.
There are various other radiometric dating methods used today to give ages of millions or billions of years for rocks.
Unless this effect (which is additional to the magnetic field issue just discussed) were corrected for, carbon dating of fossils formed in the flood would give ages much older than the true ages.
Creationist researchers have suggested that dates of 35,000 - 45,000 years should be re-calibrated to the biblical date of the flood. Such a re-calibration makes sense of anomalous data from carbon dating—for example, very discordant “dates” for different parts of a frozen musk ox carcass from Alaska and an inordinately slow rate of accumulation of ground sloth dung pellets in the older layers of a cave where the layers were carbon dated. Also, volcanoes emit much COC.